This invention pertains generally to frequency synthesizers, and more particularly to a device for generating spectrally pure signals of selected frequencies that may be any arbitrary rational fraction of a reference frequency.
It is known in the art to generate frequencies by means of modulo-N counters, which may be set to produce one output pulse for a selected number of pulses of the reference frequency. Signals derived from these output pulses may have a high degree of spectral purity, since the time period between output pulses is relatively constant, but only frequencies that are exact subharmonics of the reference frequency may be generated by prior art devices employing this technique.
Frequencies other than subharmonics of the reference frequency have been generated by prior art frequency sources using rate multipliers, such as an SN 7497 integrated circuit package, which produces a programmed number of output pulses for a fixed number of input pulses. Rate multipliers produce output pulses at an average frequency which is any rational fraction of the reference frequency, but, since output pulses of a rate multiplier must coincide with the occurrence of an input pulse, the periods between successive output pulses may not be constant, and the resultant output signal will have poor spectral purity.
Other prior art precision frequency sources have employed, for example, phase-locked loops to control the selected output frequency. These prior art sources are disadvantageous in that extensive and complex analog circuitry is required.
Accordingly, it is an object of the present invention to provide a circuit for generating an output signal at a frequency that is any arbitrary fraction of a reference oscillator frequency. Further objects of this invention are to provide a means for producing output signals having high spectral purity by eliminating cycle-to-cycle variations in the occurrence of pulses of the output signal, and to provide such signals with a minimum of analog circuitry.
These objects are accomplished in accordance with the preferred embodiment of the invention by the use of a fixed reference oscillator, a cycle swallower, a modulo-N counter, a programmable delay generator, and an accumulator to produce output pulses spaced at intervals corresponding to a computed rational number of periods of the reference oscillator frequency. The combination of the modulo-N counter, the accumulator, and the cycle swallower allows selection of an output frequency which is any rational fraction of the reference oscillator frequency. The programmable delay generator, in conjunction with the accumulator, provides an interpolation function that equalizes the time between output pulses so that the fundamental component of the output signal of selected frequency has high spectral purity. This equalization is accomplished by delaying pulses from the modulo-N counter by a selected amount of time which is determined by the accumulator.
The selected frequency is synthesized by first calculating the ratio of the reference oscillator frequency to the selected output frequency. This calculated ratio is the period of the output frequency expressed as a number of periods of the reference oscillator frequency. The integer portion of this calculated ratio controls the modulo-N counter, which divides down the reference oscillator frequency to the subharmonic of the reference oscillator frequency nearest to the selected output frequency. The accumulator, which is a device that sums its existing contents with an incrementing number determined by the fractional portion of the aforementioned ratio calculation in response to a clocking signal, controls the length of the delay of a pulse from the modulo-N counter through the delay generator. A sum that exceeds the capacity of the accumulator generates a signal that activates the cycle swallower, thus inhibiting one pulse from the reference oscillator, so that frequencies other than exact subharmonics of the reference frequency may be generated.